Universe-shaking collision of black hole and neutron star could upend our understanding of monster cosmic mergers

The universe-shaking collision of a black hole and a neutron star just led astronomers to a strange type of orbital interaction never seen before, and it's forcing them to rethink their theories.

Before the two extremely dense objects crashed and combined, they first swooped around each other in an eccentric, oval shape resembling the swirls of a Spirograph, scientists reported March 11 in The Astrophysical Journal Letters.

The new discovery challenges the prevailing assumption about how black holes and neutron star systems form, and whether they must fall into perfectly circular orbits before they die, according to the study authors.

"The fact that this system is still eccentric at the very end of its life is essentially a smoking‑gun signal that at least some neutron star-black hole binaries must form differently [than theory predicts]," study co-author Patricia Schmidt, an associate professor of physics and astronomy at the University of Birmingham in the U.K., told Live Science in an email. This observation "forces us to rethink where, and under what conditions, these systems arise."

Einstein's ripples

In January 2020, scientists detected the first compelling evidence of a black hole swallowing a neutron star — the ultradense, collapsed core of a once-massive star — resulting in the creation of a new black hole with roughly 13 times the mass of Earth's sun.

Although the event occurred roughly a billion light-years from Earth, the researchers measured the properties of the two objects using a pair of gravitational waves. These ripples in space-time are released by extreme cosmic collisions and were first predicted by Einstein's relativity. Researchers detected the two waves, which arrived 10 days apart, using the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the United States, comprising two gravitational wave detectors separated by 1,900 miles (3,000 kilometers). The first wave, labeled GW200105, is the focus of the new study.

Using a new model developed by the University of Birmingham's Institute of Gravitational Wave Astronomy, as well as complementary data from the Virgo interferometer gravitational wave detector in Italy, the team refined their measurements of the space-time ripple and found that some initial assumptions were wrong. For example, the earlier studies of GW200105 underestimated the black hole's mass while overestimating the neutron star's mass. Those values have now been corrected.

More importantly, prior studies also assumed a perfectly circular orbit for the black-hole-neutron-star system leading up to the collision, as is often the case in pairs like these. The new research rules out that possibility with 99% certainty — also throwing the system's origins into question.

The circle is broken

Black holes and neutron stars both form when once-mighty stars exhaust their fuel and collapse into dense remnants. Under certain circumstances, two remnants can fall into a shared, binary orbit that slowly pulls the objects toward a catastrophic collision.

"Canonically, neutron star-black hole binaries are thought to form from pairs of isolated massive stars that evolve together until one becomes a black hole and the other a neutron star," Schmidt told Live Science. "However, this formation pathway predicts that by the time the objects are close enough for LIGO and Virgo to detect them, their orbit should be almost perfectly circular. An eccentric orbit at such small separations is therefore very difficult to reconcile with this standard scenario."

To paint a clearer picture of the doomed system's orbit, the new analysis looked at two underexplored properties: eccentricity (how oval the system's orbit was, like the elliptical orbit of the moon around Earth) and precession (how the rotational axis of an object changes or wobbles over time). This was the first time scientists analyzed both properties at once in a merger of a black hole and a neutron-star, according to the researchers.

The team found that the system's orbit was highly eccentric (oval-shaped), but there was no compelling evidence of precession. According to the team, this means the system's oddly egg-shaped orbit had nothing to do with changes in its rotational axis. Rather, it was most likely imprinted on the system long before its death — likely due to the gravitational pull of other objects in its environment.

"The orbit gives the game away," study co-author Geraint Pratten, a Royal Society University research fellow at the University of Birmingham, said in a statement. "Its elliptical shape just before merger shows this system did not evolve quietly in isolation but was almost certainly shaped by gravitational interactions with other stars, or perhaps a third companion."

A "new window" into the universe

This evidence of an oval-shaped orbit is a first among black-hole-neutron-star systems.

While the exact mechanism behind it remains a mystery, its mere existence proves there is no one-size-fits-all explanation for how these systems form and points to a freshly opened gap in our understanding of these extreme objects.

Narrowing that gap will require new models based on more unusual gravitational wave signals from across the universe. Finding those faint signals may require new technology, such as the forthcoming space-based Laser Interferometer Space Antenna (LISA) detector, currently under construction.

"Future gravitational‑wave detectors, both on the ground and in space, will open an entirely new window on the universe," Schmidt concluded. "They will be far more sensitive than current instruments, allowing us to detect fainter and more distant sources, and even completely new types of gravitational‑wave signals that are beyond our reach today."

Editor's note: This article was updated March 11 at 10:15 a.m. to link to the published study. It was further updated on March 23, to replace an image of the Virgo detector that was erroneously described as the LIGO detector. The distance between the two LIGO facilities has also been clarified.

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